The present invention relates generally to image generation and in particular to systems and methods for compensating output luminosity of an image on a pixel-by-pixel basis.
The appearance of an image displayed by a display device (e.g., a monitor or projection system) depends in part on environmental factors not inherent in the image generation system or the display device. For example, consider an image projected onto a screen by a projector. If the optical axis of the projector is not normal to the screen, the shape and brightness of resulting image will typically be distorted because one edge of the image is farther from the projector than the other: the image may be trapezoidal instead of rectangular, and areas of the image farther from the projector may be dimmer than areas closer to the projector. Such distortions commonly occur with tabletop projectors, where precise alignment with a screen is difficult to achieve.
As another example, ambient lighting can also affect the appearance of a displayed image. As is generally known, visibility of the displayed image is improved by increasing the brightness of the display device when the ambient lighting is strong and decreasing the brightness when the ambient lighting is dim. Many display devices provide a brightness control for making such adjustments. Ambient lighting, however, is not necessarily uniform over a surface on which the image is to be displayed. For example, a light source placed to one side of a display surface (e.g., monitor or screen) will illuminate one side of the surface more than the other. As another example, the ambient light may be partially occluded, creating shadows on part of the display surface. In such situations, portions of the image displayed on more dimly lit portions of the display surface generally appear to be brighter than other portions of the image. A standard brightness control does not address such nonuniformity, and changing the ambient lighting to uniformly illuminate the display device is not always practical.
Various techniques have been developed for addressing problems of nonuniform illumination. For instance, some existing projectors are equipped with corrective optics that can be used to equalize the effective optical path lengths of light directed at different areas of the screen and/or filters that can be inserted into the optical path to change the relative brightness of different sections of the displayed image. Such corrective optics, however, add considerably to the cost of the projector and are generally effective over only a limited range of misalignment angles. Some monitors also provide intensity adjustments to the analog signal, e.g., on a per-scanline basis. For instance, in a CRT monitor, the intensity of an electron beam can be modulated per scanline to reduce nonuniformity. This generally requires additional control circuits that increase complexity and cost of the devices.
It would therefore be desirable to correct the brightness of an image to account for nonuniformities in the display device and/or environmental factors in an economical and flexible manner.